The present invention relates to a vacuum pump which is used in, for example, a semiconductor fabrication process.
In a semiconductor fabrication process, a vacuum pump discharges a generated reaction product (gas) from a semiconductor process system. The vacuum pump has a housing where a pump mechanism is accommodated. An exhaust-passage forming portion to be connected to an exhaust-gas process system is protrusively provided outside the housing. The gas that has been exhausted from the pump mechanism is led to the exhaust-gas process system via an exhaust passage formed in the exhaust-passage forming portion.
As the exhaust-passage forming portion is not easily influenced by the heat from the pump mechanism and is thin, its temperature is lower than the temperature of the housing. Therefore, a reaction product discharged from the pump mechanism is cooled and solidified at the time it passes the exhaust-passage, and may adhere to the inner wall of the passage. If a large amount of a reaction product adheres to the inner wall of the exhaust passage, the adhered portion functions as the restriction of the gas passage, thus lowering the performance of the vacuum pump.
Particularly, that portion of the exhaust-passage forming portion which is located upstream of the gas passage is close to the connection position to the pump mechanism (the exhaust port of the pump mechanism), so that the portion is influenced by the heat and becomes relatively hot. Meanwhile, because that portion of the exhaust-passage forming portion which is located downstream of the gas passage is far from the connection position to the pump mechanism, its temperature becomes lower than the temperature of the upstream-side portion. Therefore, adhesion of a reaction product to the inner wall of the exhaust passage is more likely to occur at the downstream side portion than at the upstream side portion.
To overcome the problem, a technique of increasing the temperature at the portion where the solidification of a reaction product is likely to occur has been proposed. For instance, Japanese Laid-Open Patent Application No. 8-78300 discloses a technique which uses a heater to rise the temperature at the portion where the solidification of a reaction product is likely to occur (prior art 1).
Japanese Laid-Open Patent Application No. 8-296557 discloses a technique which efficiently transmits heat generated by the pump mechanism to the portion where the solidification of a reaction product is likely to occur by making the housing of an aluminum-based metal which has an excellent thermal conductance (prior art 2).
Japanese Laid-Open Patent Application No. 1-167497 discloses a technique of providing a heat pipe at the portion where the solidification of a reaction product is likely to occur (prior art 3).
The prior art involve the following problems.
In the case of the prior art 1, provision of a heater requires separate power supply equipment, which would lead to an increase in the equipment cost of the semiconductor fabrication process. In addition, the running cost would increase by the required generation of heat by the heater.
In the case of the prior art 2, a highly corrosive gas (e.g., ammonium chloride) is handled in the semiconductor fabrication process. Making the housing of an aluminum-based metal having a low corrosion resistance reduces the durability of the vacuum pump. Further, as the aluminum-based metal has a larger thermal expansion coefficient than, for example, an ion-based metal, the clearances of the individual sections may vary significantly, resulting in a possible gas leakage.
In the case of the prior art 3, an attempt to increase the thermal conductance of the heat pipe requires that the heat pipe should be made of an aluminum-based metal, brass or the like. This would bring about the same problem as that of the prior art 2. Because a gas flows in the hollow portion of the heat pipe, i.e., because the heat pipe forms the gas passage, the inside diameter or the like of the heat pipe should be processed accurately, resulting in a cost increase.